Memory sense amplifiers and memory verification methods

ABSTRACT

Memory sense amplifiers and memory verification methods are described. According to one aspect, a memory sense amplifier includes a first input coupled with a memory element of a memory cell, wherein the memory element has different memory states at different moments in time, a second input configured to receive a reference signal, modification circuitry configured to provide a data signal at the first input from the memory element having a plurality of different voltages corresponding to respective ones of different memory states of the memory cell at the different moments in time, and comparison circuitry coupled with the modification circuitry and configured to compare the data signal and the reference signal at the different moments in time and to provide an output signal indicative of the memory state of the memory cell at the different moments in time as a result of the comparison to implement a plurality of verify operations of the memory states of the memory cell at the different moments in time.

RELATED PATENT DATA

This application claims priority to U.S. Provisional Patent ApplicationSer. No. 61/874,861 filed Sep. 6, 2013, entitled “Memory SenseAmplifiers and Memory Verification Methods”, the disclosure of which isincorporated herein by reference.

TECHNICAL FIELD

Embodiments disclosed herein pertain to memory sense amplifiers andmemory verification methods.

BACKGROUND

Memory devices are widely used in electronic devices, such as digitalcameras and personal audio players, for storing digital data. Manydifferent types of memory are available, each using a differentfundamental technology for storing data, and the memory may be volatileor non-volatile memory. Resistive random-access memory (RRAM),conductive-bridge random-access memory (CBRAM) and flash are examples ofnon-volatile memory.

At least some embodiments described below are directed towardsverification operations of memory sense amplifiers and memoryverification methods. Some of the disclosed embodiments provideincreased speed of verification operations compared with other memoryarrangements.

BRIEF DESCRIPTION OF THE DRAWINGS

Example embodiments of the disclosure are described below with referenceto the following accompanying drawings.

FIG. 1 is a functional block diagram of a memory system according to oneembodiment.

FIG. 2 is an illustrative representation of a memory cell according toone embodiment.

FIG. 3 is a graphical representation of plural memory states of a memorycell according to one embodiment.

FIG. 4 is a schematic representation of a plurality of memory cellsaccording to one embodiment.

FIG. 5 is an illustrative representation of a tile of a memory chipaccording to one embodiment.

FIG. 6 is a schematic representation of a sense amplifier according toone embodiment.

FIG. 7 is a schematic representation of modification and supplycircuitry according to one embodiment.

FIG. 8 is a schematic representation of reference circuitry according toone embodiment.

FIG. 9 is a schematic representation of reference circuitry according toone embodiment.

FIG. 10 is a schematic representation of equalization and pre-chargecircuitry according to one embodiment.

FIG. 11 is a timing diagram of equalization and pre-charge pulsesaccording to one embodiment.

FIG. 12 is a schematic representation of comparison circuitry accordingto one embodiment.

DETAILED DESCRIPTION OF EXAMPLE EMBODIMENTS

This disclosure is submitted in furtherance of the constitutionalpurposes of the U.S. Patent Laws “to promote the progress of science anduseful arts” (Article 1, Section 8).

At least some embodiments of the disclosure are directed towards memorysense amplifiers and memory verification methods. When programmingmemory, a plurality of data values may be accessed which are to bewritten into the memory. Thereafter, memory states of the memory cellsare attempted to be written or programmed in accordance with theaccessed data values. However, one or more of the memory cells may notbe programmed in accordance with the intended data values for variousreasons (i.e., the one or more memory cells do not have the correctmemory states corresponding to the desired data values). Accordingly,verification operations may be used to verify the memory states of thememory cells following programming. In one embodiment, a data valuewhich was intended to be written into a given memory cell may bereferred to as a target data value or target memory state. According tosome embodiments described below, the specific target data value ortarget memory state for a given memory cell may be used to configure asense amplifier in one of a plurality of different configurations whichcorresponds to the specific target data value/memory state to beverified and which optimizes the sense amplifier to sense the specifictarget data value/memory state.

Referring to FIG. 1, a functional block diagram of a memory system 10 isshown according to one embodiment. The illustrated memory system 10includes a controller 12, access circuitry 14, and memory 16. Memorysystem 10 may be implemented within or with respect to variousassociated devices (not shown), such as computers, cameras, mediaplayers, and thumb drives, in some examples. Memory system 10 storesdata generated or utilized by the associated devices in the describedexamples. Other embodiments of memory system 10 are possible and mayinclude more, less and/or alternative components or circuitry.

Controller 12 controls operations of writing, reading and re-writingdata of memory 16 as well as interfacing with other components orcircuitry, such as sources of data to be stored within memory 16.Controller 12 may access and process commands with respect to memory 16during operations of an associated device. Example commands instruct thegeneration of program (reset) and erase (set) voltage potentials whichare applied to memory 16 in one embodiment. The program and eraseoperations are used to write data to memory (i.e., program the memory)and are both referred to as write operations in one embodiment.Controller 12 may also control the application of read and verify pulsesto memory 16 to read and verify stored data in one embodiment.

In one embodiment, controller 12 is configured to process data, controldata access and storage, issue commands, and control other desiredoperations. Controller 12 may comprise processing circuitry configuredto execute programming provided by appropriate computer-readable storagemedia (e.g., memory) in at least one embodiment. For example, thecontroller 12 may be implemented as one or more processor(s) and/orother structure configured to execute executable instructions including,for example, software and/or firmware instructions. Other exampleembodiments of controller 12 may include hardware logic, PGA, FPGA,ASIC, state machines, and/or other structures alone or in combinationwith one or more processor(s). These examples of controller 12 are forillustration and other configurations are possible.

Access circuitry 14 is coupled with controller 12 and memory 16 and isconfigured to implement addressing (selection of columns and rows ofmemory 16), writing, reading, verifying and re-writing operations withrespect to memory cells of memory 16 in one embodiment. For example,access circuitry 14 may receive instructions from controller 12 toselect a specific block, page, word or byte of the memory 16 as well asto implement writing, reading, verifying and re-writing with respect toa plurality of cells of the selected block, page, word or byte. Asdiscussed below, the access circuitry 14 may apply electrical voltagepotentials to the memory 16 to perform write, read and verificationoperations in one embodiment.

Memory 16 includes a plurality of memory cells configured to store data,conductors electrically connected with the memory cells, and perhapsadditional circuitry, for example circuits of the access circuitry 14.At least some of the memory cells are individually capable of beingprogrammed to a plurality of different memory states at a plurality ofmoments in time. Memory 16 is accessible to the user and/or associateddevice for storage of digital information. The memory cells may beconfigured as non-volatile cells in some implementations and may havedifferent electrical resistances corresponding to different memorystates. In one specific example implementation, memory 16 is implementedas conductive bridge random access memory (CBRAM) and the memory cellsare conductive bridge memory cells.

Memory 16 may be implemented in different arrangements in differentembodiments. For example, the memory 16 may be implemented within amemory device, such as a chip, a portion of the chip (e.g., tiles and/orsub-tiles discussed below) or other arrangements. The memory device mayalso include controller 12 and/or access circuitry 14 or portionsthereof.

Referring to FIG. 2, an example of a memory cell 20 of memory 16 isshown. The illustrated example memory cell 16 is a one transistor/oneresistor (1T1R) CBRAM memory cell. Other types of memory cells may beutilized in other embodiments. For example, the disclosed senseamplifiers and verification operations may also be utilized to senseReRAM, PCM, MRAM, Spin RAM, and flash memory (1T, 1R, 1T1R, and 1D1R).

The example memory cell 16 includes a top electrode 22, memory element21 and bottom electrode 24. Top and bottom electrodes 22, 24 compriseelectrically conductive material and may also be referred to as firstand second electrodes (or vice versa) of the memory cell 20 in oneembodiment.

The illustrated embodiment of memory element 21 includes an electricallyconductive source member or layer 26 and a dielectric layer 28intermediate the electrodes 22, 24. In one embodiment, the source layer26 is a Cu+ source layer (e.g., CuTe), example materials of thedielectric layer 28 include AlOx, HfOx, and ZrOx, and the bottomelectrode 24 is titanium nitride (TiN). Other embodiments are possible.Top electrode 22 may be coupled with or part of a conductive commonsource line or plate as discussed in further detail below.

The memory cell 20 shown in FIG. 2 includes one or more conductivestructures 29 (e.g., filaments) in a low resistance state which maycorrespond to one of a plurality of different memory states (e.g., a“one” or “zero” in an example binary application) of the memory cell 20.The memory cell 20 may also be programmed to a high resistance statewhere the conductive structures 29 are removed and not present and whichmay correspond to another of the different memory states. Differentwrite voltage potentials may be applied across top and bottom electrodes22, 24 to change the resistance (and memory state) of the memory cell20.

More specifically, an erase/set programming operation may be performedby the application of a voltage potential/bias to the top electrode 22which is more positive than the voltage potential/bias applied to thebottom electrode 24. The application of these signals causes inducementof Cu ions into dielectric layer 28 and formation of one or moreelectrically conductive structures 29 (e.g., filaments) throughdielectric layer 28 and between conductive source layer 26 and bottomelectrode 24. The formation of the structures 29 provides the memorycell 25 in a low resistance state. In one embodiment, the structures 29comprise material (e.g., copper) from the source layer 26.

A memory cell 20 having the conductive structures 29 may be programmedin a program/reset operation to a high resistance state by theapplication of a voltage potential/bias to the bottom electrode 24 whichis more positive than the voltage potential/bias applied to the topelectrode 22. The application of these signals cause Cu ions to returninto source layer 26 and dissolves any electrically conductivestructures 29 within dielectric layer 28, thereby increasing theelectrical resistance of the memory element 21 between the electrodes22, 24 and providing the memory cell 20 in a high resistance state.

Memory cell 20 being may be repeatedly written between the high and lowresistance arrangements at different moments in time to store differentdata values corresponding to the different memory (e.g., resistive)states. In one embodiment, a current is passed through the memory cell22 and sense circuitry may measure the current to determine theresistance and memory state of the memory cell 20.

FIG. 2 also illustrates an access transistor 30 (e.g., NMOS) having agate 32 coupled with a word line 34 and plural terminals coupled withbottom electrode 24 and a bit line 36. Word line 34 is used to selectthe memory cell 20 for reading/writing/verification and bit line 36 isused to conduct appropriate signals for the reading/writing/verificationof the memory cell 20.

FIG. 3 illustrates an IV curve of an example 50 nm CBRAM memory cell 20in a voltage sweeping mode wherein the voltage polarity across the cellin a set/reset operation is defined as plus/minus, respectively. Asshown, the memory cell is provided in a high resistive state (HRS)during a reset operation and is provided in a low resistive state (LRS)during a set operation.

Referring to FIG. 4, a plurality of memory cells 20 are coupled with aplurality of bitlines 36, wordlines 34, and plate electrode 22. Otherarrangements of the memory cells 20 are possible.

Referring to FIG. 5, a tile 40 of a memory device is shown according toone embodiment. The memory device may comprise a memory chip in oneembodiment and which may include a plurality of tiles 40 (e.g., 16 tilesin the illustrated example).

The depicted tile 40 includes a memory array 42 of a plurality of memorycells 20 which may be individually addressed by WL drivers 44 and Y-MUXcircuitry 45. The tile 40 additionally includes an LIO controller 46,Vcommon driver 47, RRef 48, write driver 49 and a sense amplifier 50 inthe illustrated embodiment. Tile 40 includes sixty-four of individualcircuits 48, 49 and 50 to interface with a plurality of memory cells 20of array 42 in parallel in one embodiment. LIO controller 46 providesinterfacing of the sense amplifiers 50 of a given bank of the tile 40 toa databus (not shown) which is shared between multiple banks and alsointerfaces with an I/O block of the memory chip. Plate voltage driver 47drives the plate voltage to the various voltage values utilized forreading and writing. The write driver 49 drives the bitline voltage tothe various voltage values utilized for writing. Sense amplifiers 50sense the memory states of memory cells 20 during read and verificationoperations. Additional details and operations of sense amplifiers 50 aredescribed below in example embodiments.

It is desired in some embodiments to increase the speed of verificationoperations. In one embodiment, set and reset cells are verified at thesame time using multiple sense amplifiers in parallel which is fasterthan verifying set and reset cells in series. Set and reset cells may beverified in series in other embodiments. The time or speed for theexample configuration of the sense amplifier discussed below withrespect to FIG. 6 to sense the cell current is proportional to themagnitude of the current. As discussed above, memory element 21 ofmemory cell 20 is configured to have different electrical resistances indifferent memory states. For example, memory element 20 has a relativelyhigh resistance in a reset state (also referred to as high resistivestate or HRS) compared with a set state (also referred to as a lowresistive state or LRS).

As discussed below, some embodiments of the sense amplifiers 50 of thememory device perform operations to increase the speed during verify ofreset memory cells. Furthermore, reset verify operations may beperformed at the same time as (in parallel with) set verify operationseven though the memory cell has a higher electrical resistance in thereset state than during the set state. In some embodiments, the targetdata value (0, 1) or corresponding target memory state (HRS or LRS)which was attempted to be written to the respective memory cell 20 beingverified may be used to implement the operations including tailoring thesense amplifier 50 to sense the cells currents of the target memorystate.

Referring to FIG. 6, one embodiment of sense amplifier 50 is shown. Theillustrated sense amplifier 50 includes plural inputs 51, 52 whichreceive a data signal (corresponding to the cell current) and areference signal, respectively. In particular, input 51 is coupled withbitline 36 and input 52 is coupled with reference circuitry 57configured to provide one or more reference signals as discussed furtherwith respect to example embodiments below. Although only one memory cell20 is shown in FIG. 6, typically numerous additional memory cells 20 mayalso be coupled with bitline 36 and which are individually addressableto be sensed via respective wordlines 34.

The sense amplifier 50 additionally includes modification circuitry 53coupled with the first and second inputs 51, 52 and amplification andcomparison circuitry 54 coupled with modification circuitry 53. In oneembodiment, modification circuitry 53 is configured to provide the datasignal at a regulated voltage for sense operations including cellverification operations. As discussed further below, modificationcircuitry 53 may be implemented using cascade circuitry in a morespecific example embodiment. Furthermore, reference circuitry 57 iscoupled with second input 52 as mentioned above, supply circuitry 58 isconfigured to supply one or more voltage biases during verificationoperations as discussed in additional detail below, and control circuit59 is configured to control operations of the sense amplifier 50 (forexample using a target data value or memory state) as also discussed inadditional detail below.

In one embodiment, sense amplifier 50 provides comparison operations ofthe data signal with the reference signal during sense operations andincludes a plurality of paths, namely a data path 61 and a referencepath 62. However, in one embodiment, and prior to the comparison,modification circuitry 53 provides the data and reference signals atdesired voltage values according to the target data value which waspreviously attempted to be written to the memory cell 20 and which is tobe verified. In one embodiment, modification circuitry 53 is configuredto provide the data signal and reference signal at different voltagescorresponding to respective ones of different memory states of thememory cell to be verified as discussed in additional below.

In the illustrated embodiment, modification circuitry 53 includes aplurality of cascode transistors 70, 71 individually within one of therespective data path 61 and reference path 62. Cascode transistors 70,71 set the voltages of the data signal and reference signal atrespective inputs 51, 52 where input 51 is the bitline and the voltageof input 51 may be referred to as the bitline voltage.

Still referring to FIG. 6, supply circuitry 58 provides a first voltagebias vcasc(set) to the gates of cascode transistors 70, 71 during setverify operations and provides a greater, second voltage biasvcasc(reset) to the gates of cascode transistors 70, 71 during resetverify operations. The provision of the different gate bias voltagesprovides the data signals and reference signals with different voltagesduring different memory states of the memory cell.

As mentioned previously, the current of the data signal (also referredto as cell current) for a reset verify operation is lower than thecurrent of the data signal for a set verify operation due to theelectrical resistances of the memory cell 20. Some of the exampleembodiments discussed below increase the speed of verificationoperations of reset cells (which otherwise would be slower due to thereduced currents of the memory cell in the reset (high resistive state))thereby increasing the speed of the overall verification operations ofthe memory device. Furthermore, in some embodiments, the verification ofthe reset cells may be performed in parallel with the verificationoperations of set cells. In one embodiment, the target state of thememory element or cell (i.e., the state or value which was attempted tobe written to the respective memory element or cell) is used toimplement the reset and set verification operations and to increase theverify sensing speed of reset cells by the sense amplifier 50.

In one embodiment, the target state of the memory element or cell isused to determine which of the plurality of voltage biases is used forset and reset verification operations. For example, a data latch ofcontrol circuit 59 may indicate which data value or memory state thememory cell 20 should be placed into following the previous writeoperation. This value is used to control one or more operations of thesense amplifier 50 to improve the verification speed with respect toreset cells as discussed in detail below.

In one more specific embodiment, the data of the memory cell (set orreset) is used to select an appropriate one of plural bias voltages forcascode transistors 70, 71 for implementing set and reset verificationoperations. The cascode transistors 70, 71 set the respective SEN andREN bitline voltages and are each approximately the gate voltage minusthe threshold voltage of the respective transistor 70, 71. As discussedabove, the cell current is less when the memory cell is in the highresistive state compared with the low resistive state. When attemptingto verify a reset memory cell (HRS), it is desired to provide anincreased gate bias voltage to the cascode transistors 70, 71 comparedwith the gate bias voltage used to verify a set memory cell (LRS). Morespecifically, the higher gate bias voltage provides the bitline voltageat input 51 at an increased voltage compared with the bitline voltagewhich would result from the use of the bias voltage which is the same asused during a set verify operation. The application of the increasedbias voltage (e.g., vcasc(reset)=1 V) to the gates of the transistors70, 71 results in the data and references signals each having a highervoltage of approximately 500 mV compared with the voltage of 200 mV ifthe lower bias voltage (e.g., vcasc(set)=700 mV) was used. Accordingly,the data signals and reference signals have different characteristicscorresponding to different states of the memory cell in one embodiment.

The use of the increased voltage results in the data signal havingincreased current for reset verify (e.g., 1.2 microA) which enablesincreased sensing speed of the sense amplifier 50 (since sense speed isproportional to the current) and compared with use of the lower voltagebias which would result in the data signal having a smaller current(e.g., 0.15 microA).

It is noted that the use of increased voltages during read operationsmay disturb the state of the respective memory cell, however, for verifyoperations the memory cell is not exposed to the increased voltageprovided by the increased bias voltage for a prolonged period of time orfor numerous cycles (e.g., millions of read cycles) and the state of thememory cell being verified is not disturbed.

Accordingly, in one embodiment, supply circuitry 58 is configured toprovide the vcasc(reset) voltage bias for verifying a reset memory stateand which has a larger voltage bias than the voltage bias of vcasc(set)which is used for verifying a set memory state. If the data latchindicates that a set data value was to be written as the target, thenswitch 80 is closed to apply the vcasc(set) voltage bias to the gates ofthe cascode transistors 70, 71 while switch 81 is closed to apply thevcasc(reset) voltage bias to the gates of the cascode transistors 70, 71if the data latch indicates that a reset data value was to be written asthe target in the presently described embodiment. The application of theincreased voltage bias to the cascode transistors 70, 71 during resetverify results in the data and reference signals having increasedvoltages and current during reset verify operations compared with thearrangements which use the same bias voltage for reset verify operationsas the set verify operations.

The different cell currents in the high and low resistance states of thememory cell are significantly different which presents problems withrespect to sensing of cell currents in both memory cell states by thesense amplifier 50. In particular, the sense amplifier may be slow whileperforming reset verify operations due to the smaller reset cellcurrent. At least some of the embodiments above provide apparatus andmethods to collapse the difference of the cells currents in the high andlow resistance states of the memory cell to be closer to one another andwithin a smaller current range for sensing by the sense amplifier. Forexample, the use of the different cascode gate voltages duringrespective set verify and reset verify operations provides the datasignals having currents of approximately 1.5 and 1.2 microA as discussedabove compared with arrangements which use the same cascode gate voltageduring respective set verify and reset verify operations and resultingin currents which may be 10× different from one another. Accordingly,the sense amplifier 50 may be optimized to sense on a smaller range ofcurrents in some embodiments.

According to one embodiment, reference circuitry 57 is configured toprovide a plurality of different reference signals to second input 52corresponding to the different target data written to the respectivememory cell 20 which provides additional flexibility during verificationoperations compared with arrangements which use a constant referencecurrent. In one embodiment, reference circuitry 57 includes pluralcurrent sources 77, 78 which provide reference signals having differentcurrents. For example, in one embodiment described above, currentsources 77, 78 may provide currents corresponding to expected cellcurrents of memory cell 20 in both low and high resistive states (e.g.,1.5 microA and 1.2 microA), respectively. If control circuit 59indicates that the memory cell 20 should be in a set state according tothe target, then switch 82 may be closed to couple current source 77with second input 52 to provide a reference current of 1.5 microA. Ifcontrol circuit 59 indicates that the memory cell 20 should be in areset state according to the target, then switch 83 may be closed tocouple current source 78 with second input 52 to provide a referencecurrent of 1.2 microA. In another embodiment, reference circuitry 57provides a constant reference current or reference signals having othercurrents than those described above.

The data signal and reference are provided to amplification andcomparison circuitry 54 which amplifies and compares the data andreference signals from the cascode transistors 70, 71 with one anotherto generate an output signal at output 55 which may be used to determinewhether the data value is verified or not. Circuitry 54 includes PMOStransistors 72, 74 arranged in a current mirror configuration in theillustrated embodiment and which may be referred to as current mirrortransistors. The data and reference signals at nodes sin and rin developa differential voltage which are equal if the currents of the data andreference signals are equal in one embodiment.

The differential voltage is compared using comparator 76 in the depictedembodiment which provides a logical value (e.g., 1) if the current ofthe data signal is greater than the current of the reference signal andotherwise provides another logical value (e.g., 0). Additional logiccircuitry receives the output signal and determines whether the memorycell in a target set state is verified (e.g., current of the data signalis at least 1.5 microA) or determines whether the memory cell in atarget reset state is verified (e.g., current of the data signal is lessthan 1.2 microA) in one embodiment. Additional write and/or verifyoperations may be performed if the current verify operation fails in oneimplementation.

Equalization and pre-charge operations may be implemented in someembodiments prior to comparison of the data signal and the referencesignal. In one embodiment, the equalization and pre-charge operationsbegin simultaneously when a new verify operation is to be performedfollowing a previous write or verify operation.

For the equalization operation, switches 64, 65 are closed which operateto short the sin and rin nodes of circuitry 54 and provide the senseamplifier 50 in an equalized state from a previous state. For thepre-charge operation, switches 66 shunt the current mirror transistors72, 74 by coupling the sin and rin nodes to the supply voltage and whichprovides faster bit line charging compared with arrangements which donot use pre-charge operations. While equalization and pre-charge maybegin simultaneously, the pre-charge is disabled before the equalizationoperations in one embodiment. Thereafter, the differential voltagesdevelop within the current mirror of the amplification and comparisoncircuitry 54 and which may be compared to determine whether the state ofthe memory cell is verified or not. In one embodiment, the senseamplifier 50 for a given bit may be turned off once the bit has beenverified and during additional verification operations for differentbits by other sense amplifiers 50 of the memory device.

The control lines set_fail and reset_fail are utilized to implementverification operations of set and reset target memory states of amemory cell, respectively. In one embodiment, all sense amplifiers 50are enabled for the first verify operation following a write operationand have one of their respective set_fail and reset_fail control linesasserted according to the target of the respective memory cells to beverified. In some embodiments, additional verification operations may beneeded with respect to one or more memory cells which failed to verify.Once a given memory cell has been verified, the respective senseamplifier 50 for the given memory cell may be turned off whilesubsequent verification operations of other memory cells are performedin one embodiment.

In addition, pre-read operations may be implemented in some embodimentswhere memory cells may also be verified prior to application of writepulses to the memory cells. For example, users often write random dataover previous random data and there is a high probability that the newdata bit is equal to the old data bit, and accordingly a write pulse isnot required if the new and old bits are equal.

Referring to FIG. 7, other configurations of modification circuitry 53 aand supply circuitry 58 a are shown. Modification circuitry 53 aincludes cascode transistors 70, 71 with respective op-amps 90, 91provided in a feedback arrangement for the data and reference paths andwhich regulate the voltages at inputs 51, 52. At equilibrium, thevoltages of inputs 51, 52 will be equal to Vbl_target provided by supplycircuitry 58 a where Vbl_target is the target bitline voltage (e.g., 200mV and 500 mV for set and reset, respectively). The equilibrium point ofthe feedback arrangement is the point where the two inputs of op-amp 90or 91 are at approximately equal voltage and the op-amp output voltageis the cascode gate voltage utilized to produce the desired bitlinevoltage (e.g., approximately 500 mV higher than the target bitlinevoltage).

Referring to FIG. 8, another configuration of reference circuitry 57 ais shown. Reference circuitry 57 a includes an NMOS transistor 100coupled with switches 82, 83 and input 52. Plural bias voltagesvgiref_set and vgiref_reset are selectively coupled by respectiveswitches 82, 83 to the gate of transistor 100 corresponding to whetherthe target state of the memory cell to be verified is a set state orreset state, respectively. The transistor 100 and voltage sources may beconfigured to provide different set and reset reference currents whichcorrespond to the expected currents of the data signal in the set andreset states, respectively, in one embodiment. In one embodiment,vgiref_set is larger than vgiref_reset to provide reference signalshaving currents during set verify operations which are greater than thecurrents of reference signals used during reset verify operations.

Referring to FIG. 9, yet another configuration of reference circuitry 57b is shown. Reference circuitry 57 b includes plural NMOS transistors102, 104 coupled with switches 106, 108 and input 52. Similar to thearrangement of FIG. 8, plural bias voltages vgiref_set and vgiref_resetare selectively coupled by respective switches 82, 83 to the gates oftransistors 102, 104 corresponding to whether the target data of thememory cell to be verified is a set state or reset state, respectively.

The transistors 102, 104 and voltage sources may be configured toprovide different set and reset reference currents which correspond tothe expected currents of the data signal in the set and reset states,respectively, in one embodiment. In one embodiment, vgiref_set is largerthan vgiref_reset to provide reference currents during set verifyoperations which are greater than the reference currents used duringreset verify operations.

Furthermore, the device parameters of the transistors 102, 104 may alsobe different to provide the appropriate reference currents for set andreset verify operations. In one example embodiment, transistor 102 has alarger width to length channel ratio (and lower resistance) thantransistor 104, and accordingly, transistor 102 may be used for setverify operations where the data signal current is larger than the datasignal current of reset verify operations. This arrangement of referencecircuitry 57 b allows greater flexibility over a wider range of currentsthan the arrangements of FIGS. 6 and 8 since both transistor parametersand/or the bias voltages of the transistors 102, 104 may be varied toprovide desired reference currents for set and reset verify operations.

Referring to FIG. 10, pulses having different parameters may be used toimplement equalization and pre-charge operations during set and resetverify operations in one embodiment. In particular, the parameters(e.g., pulse widths) may be tailored to provide optimized senseoperations based upon the target data being set or rest. For example,selection circuitry 108 controls the use of a first group of equalizeand pre-charge pulses for a set verify operation and a second group ofequalize and pre-charge pulses for a reset verify operation.

Referring to FIG. 11, a timing diagram of various signals includingequalization and pre-charge pulses for the arrangement of FIG. 10 areillustrated according to one embodiment. The top group 109 of signalsillustrates the signals which are input to selection circuitry 108. Themiddle group 110 of signals illustrates the signals which are outputtedby selection circuitry 108 and applied to the sense amplifier forimplementing a set verify operation. The bottom group 111 of signalsillustrates the signals which are outputted by selection circuitry 108and applied to the sense amplifier for implementing a reset verifyoperation. The signals of groups 110, 111 are selected to optimize thesensing of the sense amplifier 50 for set verify and reset verifyoperations, respectively.

More specifically, the sense amplifier's bitline charging time isdependent on cell current in one embodiment. For a given total sensingtime (e.g., pre-charge+equalization+amplification) the accuracy of thesense amplifier improves somewhat if, as cell current is reduced, thepre-charge and equalization times are increased. In one embodiment, theamplification phase is delayed until after the bitline is fully charged,and bitline charging takes a longer period of time for reduced cellcurrent. Even though the amplification time is reduced (which reducesaccuracy) the overall accuracy increases because of the provision of afully charged bitline in one example embodiment.

Referring to FIG. 12, another embodiment of amplification and comparisoncircuitry 54 a is shown. This illustrated embodiment is useful where arelatively wide range of input currents and/or relatively small currentsmay be compared. This embodiment includes a plurality of current mirrortransistors 112, 114, 116, 118 for providing current mirror operationsas previously described. In addition, circuitry 54 a includes switches120, 122 which are configured to select which of the transistors 112,114, 116, 118 are used for verification of an individual bit based uponthe target memory state.

For example, the device parameters (e.g., channel W/L dimensions) of thetransistors may be tailored for use with either set or reset verifyoperations and to reduce sensitivity of circuitry 54 a to devicemismatch of the transistors of the current mirror. The W/L channeldimensions of transistors 112, 114 used for set verify operations mayhave a larger W/L ratio (and smaller resistance) than transistors 116,118. The use of transistors 116, 118 for reset verify operations eachhaving a larger resistance than individual transistors 112, 114increases the voltage swings/differentials for comparison compared withuse of the lower resistance transistors 112, 114. Accordingly,transistors 116, 118 may be used with the lower current signals (e.g.,data and reference signals during reset verify operations) whiletransistors 112, 114 may be used with higher current signals (e.g., dataand reference signals during set verify operations).

In addition, use of higher Vgs voltages across the current mirrortransistors results in the current mirror being less susceptible tomismatch of the threshold voltages (Vth) of the current mirrortransistors since the sin and rin values are each equal to Vgs−Vth ofthe respective current mirror transistor. Furthermore, using increasedcurrents (for example provided by reference circuitry 57 and/ormodification circuitry 53) also reduces the sensitivity of the currentmirror to mismatch of the threshold voltages (Vth) of the current mirrortransistors.

CONCLUSION

In some embodiments, a memory sense amplifier comprises a first inputcoupled with a memory element of a memory cell, wherein the memoryelement has different memory states at different moments in time, asecond input configured to receive a reference signal, modificationcircuitry configured to provide a data signal at the first input fromthe memory element having a plurality of different voltagescorresponding to respective ones of different memory states of thememory cell at the different moments in time, and comparison circuitrycoupled with the modification circuitry and configured to compare thedata signal and the reference signal at the different moments in timeand to provide an output signal indicative of the memory state of thememory cell at the different moments in time as a result of thecomparison to implement a plurality of verify operations of the memorystates of the memory cell at the different moments in time.

In some embodiments, a memory sense amplifier further comprises acascode transistor configured to define a voltage of a data signal froma memory cell, and supply circuitry configured to provide a firstvoltage bias to a gate of the cascode transistor corresponding to afirst of a plurality of memory states of the memory cell to be verifiedat a first moment in time and a second voltage bias to the gate of thecascode transistor corresponding to a second of the memory states to beverified at a second moment in time.

In some embodiments, a memory verification method comprises accessing aplurality of target data values indicative of a plurality of memorystates of the memory cell at different moments in time, accessing a datasignal from the memory cell at the different moments in time, modifyingthe data signal differently corresponding to respective ones of thedifferent memory states of the memory cell at the different moments intime, and using the modified data signal, verifying the memory states ofthe memory cell at the different moments in time.

In some embodiments, a memory verification method comprises, accessingone of a plurality of target data values to be written to a memory cell,attempting to write the one target data value to the memory cell, andusing the one target data value, performing a verification operation ofa memory state of the memory cell comprising accessing a data signalfrom the memory cell, selecting one of a plurality of modifications ofthe data signal corresponding to the one target data value, convertingthe data signal using the one modification, and using the modified datasignal, determining whether the memory state of the memory cellcorresponds to the one target data value.

In compliance with the statute, the invention has been described inlanguage more or less specific as to structural and methodical features.It is to be understood, however, that the invention is not limited tothe specific features shown and described, since the means hereindisclosed comprise preferred forms of putting the invention into effect.The invention is, therefore, claimed in any of its forms ormodifications within the proper scope of the appended aspectsappropriately interpreted in accordance with the doctrine ofequivalents.

Further, aspects herein have been presented for guidance in constructionand/or operation of illustrative embodiments of the disclosure.Applicant(s) hereof consider these described illustrative embodiments toalso include, disclose and describe further inventive aspects inaddition to those explicitly disclosed. For example, the additionalinventive aspects may include less, more and/or alternative featuresthan those described in the illustrative embodiments. In more specificexamples, Applicants consider the disclosure to include, disclose anddescribe methods which include less, more and/or alternative steps thanthose methods explicitly disclosed as well as apparatus which includesless, more and/or alternative structure than the explicitly disclosedstructure.

What is claimed is:
 1. A memory sense amplifier comprising: a firstinput coupled with a memory element of a memory cell, wherein the memoryelement has different memory states at different moments in time; asecond input configured to receive a reference signal; modificationcircuitry configured to provide a data signal at the first input fromthe memory element having a plurality of different voltagescorresponding to respective ones of the different memory states of thememory element at the different moments in time, wherein themodification circuitry comprises a plurality of transistors individuallycoupled with one of the first and the second inputs; supply circuitryconfigured to provide different bias signals to the transistors, thedifferent bias signals corresponding to the different memory states ofthe memory element to be verified at the different moments in time; andcomparison circuitry coupled with the modification circuitry andconfigured to compare the data signal and the reference signal at thedifferent moments in time and to provide an output signal indicative ofthe memory states of the memory element at the different moments in timeas a result of the comparison to implement a plurality of verifyoperations of the memory states of the memory element at the differentmoments in time.
 2. The amplifier of claim 1 wherein the modificationcircuitry is configured to provide the reference signal having aplurality of different voltages corresponding to respective ones of thedifferent memory states of the memory element at the different momentsin time.
 3. The amplifier of claim 1 wherein the transistors compriseparallel cascode transistors and the comparison circuitry comprises acurrent mirror.
 4. A memory sense amplifier comprising: a first inputcoupled with a memory element of a memory cell, wherein the memoryelement has different memory states at different moments in time: asecond input configured to receive a reference signal; modificationcircuitry configured to provide a data signal at the first input fromthe memory element having a plurality of different voltagescorresponding to respective ones of the different memory states of thememory element at the different moments in time; comparison circuitrycoupled with the modification circuitry and configured to compare thedata signal and the reference signal at the different moments in timeand to provide an output signal indicative of the memory states of thememory element at the different moments in time as a result of thecomparison to implement a plurality of verify operations of the memorystates of the memory element at the different moments in time; andwherein the modification circuitry comprises parallel cascodetransistors individually coupled with one of the first and secondinputs, and further comprising supply circuitry configured to providedifferent voltage biases to a gate of one of the cascode transistors toprovide the data signal having the different voltages.
 5. A memory senseamplifier comprising: a first input coupled with a memory element of amemory cell, wherein the memory element has different memory states atdifferent moments in time; a second input configured to receive areference signal; modification circuitry configured to provide a datasignal at the first input from the memory element having a plurality ofdifferent voltages corresponding to respective ones of the differentmemory states of the memory element at the different moments in time;comparison circuitry coupled with the modification circuitry andconfigured to compare the data signal and the reference signal at thedifferent moments in time and to provide an output signal indicative ofthe memory states of the memory element at the different moments in timeas a result of the comparison to implement a plurality of verifyoperations of the memory states of the memory element at the differentmoments in time; and wherein the modification circuitry comprisesparallel cascode transistors individually coupled with one of the firstand second inputs, and further comprising supply circuitry configured toprovide a first voltage bias to one of the cascode transistorscorresponding to a first of the different memory states and to provide asecond voltage bias to the one of the cascode transistors correspondingto a second of the different memory states.
 6. The amplifier of claim 5wherein the memory element has an increased electrical resistance in thefirst memory state compared with the second memory state, and the firstvoltage bias is greater than the second voltage bias.
 7. The amplifierof claim 1 further comprising a control circuit configured to determinea plurality of target memory states to which the memory cell is to beprogrammed to at the different moments in time, and to use thedetermined target memory states to control the provision of the datasignal having the different voltages at the different moments in time.8. The amplifier of claim 1 further comprising reference circuitryconfigured to provide the reference signal comprising differentcharacteristics corresponding to the different memory states of thememory element at the different moments in time.
 9. The amplifier ofclaim 1 further comprising equalization and pre-charge circuitryconfigured to equalize and pre-charge the memory sense amplifier fordifferent lengths of time corresponding to verification of the differentmemory states of the memory element.
 10. A memory sense amplifiercomprising: a cascode transistor configured to define a voltage of adata signal from a memory cell; and supply circuitry configured toprovide a first voltage bias to a gate of the cascode transistorcorresponding to a first of a plurality of memory states of the memorycell to be verified at a first moment in time and a second voltage biasto the gate of the cascode transistor corresponding to a second of thememory states to be verified at a second moment in time.
 11. Theamplifier of claim 10 wherein the cascode transistor provides the datasignal at a first voltage as a result of the provision of the firstvoltage bias and provides the data signal at a second voltage as aresult of the provision of the second voltage bias, and wherein thefirst and second voltages are different.
 12. The amplifier of claim 11further comprising a current mirror configured to compare the voltagesignal with a reference signal at the first and second moments in timeand to provide an output signal indicative of the memory state of thememory cell at the different moments in time as a result of thecomparison.
 13. The amplifier of claim 10 wherein the memory cell has anincreased electrical resistance in the first memory state, and the firstvoltage bias is greater than the second voltage bias.
 14. The amplifierof claim 10 further comprising a control circuit configured to determinethe first and the second memory states of the memory cell to be verifiedat the first and second moments in time, and to use the determined firstand second memory states to control the provision of the first andsecond voltage biases to the gate of the cascode transistor at the firstand second moments in time.
 15. A memory verification method comprising:accessing a plurality of target data values indicative of a plurality ofmemory states of the memory cell at different moments in time; accessinga data signal from the memory cell at the different moments in time;modifying the data signal differently corresponding to respective onesof the different memory states of the memory cell at the differentmoments in time; and using the modified data signal, verifying thememory states of the memory cell at the different moments in time. 16.The method of claim 15 wherein the modifying comprises providing thedata signal having different voltages at the different moments in timecorresponding to the different target data values.
 17. The method ofclaim 16 wherein the modifying comprises modifying using a cascodetransistor.
 18. The method of claim 17 wherein the modifying comprisesapplying different bias voltages to a gate of the cascode transistorcorresponding to the different memory states.
 19. The method of claim 16wherein the data signal has a smaller current corresponding to a firstof the memory states of the memory cell compared with a second of thememory states of the memory cell, and wherein the modifying comprisesmodifying the data signal to an increased voltage corresponding to thefirst of the memory states compared with the second of the memory statesof the memory cell.
 20. The method of claim 19 wherein the memory cellhas different electrical resistances corresponding to the differentmemory states including an increased electrical resistance in the firstof the memory states compared with the second of the memory states. 21.The method of claim 16 wherein the verifying comprises: comparing themodified data signal to a reference signal; and as a result of thecomparing, providing an output signal indicative of the memory states ofthe memory cell at the different moments in time.
 22. The method ofclaim 21 further comprising, before the comparing, providing thereference signal having different voltages at the different moments intime corresponding to the different target data values.
 23. The methodof claim 15 wherein the modifying comprises modifying using a senseamplifier, and further comprising, before the verifying, equalizing andpre-charging the sense amplifier for different lengths of timecorresponding to the different memory states of the memory cell.
 24. Amemory verification method comprising: accessing one of a plurality oftarget data values to be written to a memory cell; attempting to writethe one target data value to the memory cell; and using the one targetdata value, performing a verification operation of a memory state of thememory cell comprising: accessing a data signal from the memory cell;selecting one of a plurality of modifications of the data signalcorresponding to the one target data value; converting the data signalusing the one modification; and using the modified data signal,determining whether the memory state of the memory cell corresponds tothe one target data value.
 25. The method of claim 24 wherein theplurality of modifications comprise providing the data signal atdifferent voltages.
 26. The method of claim 24 wherein the convertingcomprises converting the data signal using a cascode transistor.
 27. Themethod of claim 26 wherein the converting comprises applying differentbias voltages to a gate of the cascode transistor corresponding todifferent ones of the target data values.
 28. The method of claim 26wherein the determining comprises comparing the modified data signal toa reference signal using a current mirror.
 29. The method of claim 28further comprising, using the one target data value, selecting thereference signal from a plurality of reference signals.
 30. The methodof claim 28 further comprising, using the one target data value,selecting one of a plurality of different configurations of the currentmirror.